This invention relates to a probe having microstrip transmission lines.
An important aspect of the manufacture of integrated circuit chips is the testing of the circuit embodied in the chip in order to verify that it operates according to specification. Although the circuit could be tested after the chip has been packaged, the expense involved in dicing the wafer and packaging the individual chips makes it desirable to test the integrated circuit as early as possible in the fabrication process, so that unnecessary efforts will not be expended on faulty devices. It is therefore desirable that these circuits be tested either immediately after wafer fabrication is completed, and before separation into dies, or after dicing but before packaging. In either case, it is necessary to make electrical connection to all the circuit's external connection points (usually bonding pads) in a non-destructive way, so as not to interfere with subsequent packaging and connection operations.
Typical high-speed circuits are designed to operate with input and output signal bandwidths exceeding 1 GHz and to receive and transmit signals from and into transmission lines of predetermined characteristic impedance, typically 50 ohms. It is desirable that an integrated circuit be tested under its design operating conditions and to the extremes of its design performance range. This necessitates that high-speed circuits be tested at these high frequencies, which requires that transmission lines be employed to transmit signals into and receive signals from the circuits.
Typically, an integrated circuit is tested in a probe station by use of a test and measurement instrument that is connected to the device under test (DUT) by use of a probe. Probes that are currently commercially available comprise a substrate of insulating material and multiple conductive probe elements that are supported by the substrate. The probe elements are connected to the test and measurement instrument by coaxial cables. The probe elements have tips that are arranged in a pattern corresponding to the pattern of connection points of the DUT. The DUT is positioned in the probe station directly beneath the tips of the probe elements, and is raised to bring its connection points into contact with the tips of the probe elements.
When testing an integrated circuit, it is desirable that a good electrical connection be established between the probe elements and the connection pads of the device under test. In the event that the connection pads are gold, this requirement is generally met simply by achieving pressure contact between the connection pads and the tips of the probe elements, but if the connection pads are aluminum, it is generally necessary to penetrate a parasitic layer of aluminum oxide in order to establish a good electrically conductive connection.
An integrated circuit chip generally has many signal pads and several ground pads, and it is desirable that each ground pad be connected to a good ground at a location that is very close to the ground pad. Although the contact surfaces of the connection pads of an integrated circuit are nominally coplanar, it is necessary that a probe be able to accommodate minor departures from coplanarity.
Two types of probe that are currently commercially available are known respectively as wire probes and film probes. In a wire probe, the substrate is generally referred to as a probe card, and the probe elements are metal needles or wires that project in cantilever fashion from the probe card and are positioned with their tips in a pattern corresponding to the pattern of connection pads of the device under test.
When the device under test is raised into contact with the tips of the wires, the wires are deflected slightly and exert a scrubbing action relative to the connection pads. This ensures that any electrically insulating layer covering the connection pads is penetrated. Further, since the wires project in cantilever fashion from the probe card, their tips are deflectable independently of each other and therefore the wire probe is able to accommodate minor departures of the connection pads from coplanarity.
A disadvantage of the wire probe is that the ground element is connected to the ground plane at a point on the probe card, and therefore the ground connection cannot be any closer to the tip of the ground pad of the DUT than the length of the ground wire. Further, although the conductor runs and ground plane on the probe card form transmission lines that connect the signal elements to the external connectors, the transmission line environment ends at a distance from the device under test that is at least as great as the amount by which the probe elements project from the probe card. A further disadvantage of the wire probe is that it is necessary to use a mechanical positioner to assemble the probe, since the individual wires have to be positioned relative to each other and attached to the probe card, and use of a mechanical positioner results in the minimum spacing of the tips of the wires being rather high.
In a film probe, the substrate is a flexible film of electrically insulating material having conductor runs on one face and a ground plane on its other face. The probe elements are contact bumps that are formed directly on the conductor runs. The ground bump is connected to the ground plane through a via that interconnects the ground plane and the conductor run on which the ground bump is formed. The film probe is fabricated using photolithographic techniques, and therefore the contact bumps can be positioned with the same spacing and precision as the bonding pads of an integrated circuit.
In the film probe, the connection of the ground element to the ground plane can take place very close to the grounding pad of the DUT, and the transmission line environment can extend to a region that is very close to the connection pads of the device under test. However, the tips of the probe elements of the film probe are not able to move independently of each other to a significant extent, and therefore the film probe is not well suited to accommodate departures from coplanarity of the contact surfaces of the connection pads. Further, when the device under test is brought into contact with the tips of the probe elements of a conventional film probe, no appreciable scrubbing action takes place. Accordingly, although the film probe is electrically superior to the wire probe, the wire probe is mechanically superior to the film probe.